What If? 2: Additional Serious Scientific Answers to Absurd Hypothetical Questions (What If?, #2)

by Randall Munroe

You can feel this “cold radiation” by looking up at the stars on a summer night. Your face will feel cold since your body heat is pouring away into space. If you hold up an umbrella to block your view of the sky, you’ll feel warmer—almost as if the umbrella is “blocking the cold” from the sky.

Robin Dunbar famously suggested that the average human maintains about 150 social relationships. The total number of humans who have ever lived is somewhere north of 100 billion. A 1017-year road trip would be long enough to replay the lives of every one of those people in real time—in a sort of unedited documentary—and then rewatch every one of those documentaries 150 times, each time with a different commentary track by the 150 people who knew the subject best. By the time you finished watching this complete documentary of human perspective, you’d still be less than 1 percent of the way to the edge of the universe, so you’d have plenty of time to rewatch the whole project—each human life with all 150 commentary tracks

A spherical shell of matter doesn’t exert any gravitational force on objects inside it, which means that if you go underground, the layers of rock above you stop contributing to your weight. From a gravitational point of view, it’s as if they vanish.

Different parts of your computer are out of sync with one another. If two parts are going to go back and forth quickly, circuit board designers need to place them physically close to one another, so they’re not held back by the sluggish speed of light.

You can’t use lenses and mirrors to make something hotter than the surface of the light source itself.

The flow rate over Niagara Falls is at least 100,000 cubic feet per second, which is actually mandated by law.

There’s a neat way to answer “Will it float?” questions without doing too many complicated calculations. Water is roughly 1,000 times denser than air,10 so if you want to know whether something could float if you filled it with helium, just estimate how heavy it would be when filled with water, then move the decimal point over 3 places. That’s how much buoyancy it could produce, so it’s how light the solid parts have to be in order to float.

When a sunscreen says SPF 20, it means that it should only let in 1/20th of the Sun’s UV rays, allowing you to stay in the Sun 20 times longer before you get sunburned.